Prevention is better than cure. Here are some proactive measures to avoid encountering the steam_api_ext64.dll error again:
"An error occurred while loading steam_api_ext64.dll. The specified module could not be found." steam api ext 64dll mafia 3 full
If you bought the game on Steam, the error is likely a simple file integrity issue. Prevention is better than cure
If a game installation is interrupted due to a power outage, drive failure, or a corrupted installer package, the file may be corrupted or omitted entirely from the destination folder. The Risks of Downloading Third-Party DLL Files or a corrupted installer package
Right-click on (or Mafia III: Definitive Edition) and select Properties . Go to the Installed Files (or Local Files ) tab. Click the Verify integrity of game files button.